1. Field of the Invention
The present invention relates generally to flow passage structures for refrigerant compressors, and more particularly, to a flow passage structure that can be used in inter-stage pressure refrigerant injection for a multi-stage centrifugal refrigerant compressor.
2. Description of Related Art
Centrifugal chillers generally comprise centrifugal refrigerant compressors for compressing refrigerant. In practice, for purpose of enhancing refrigeration cycle performance and cooling capability, centrifugal refrigerant compressors having two or more stages are utilized, thereby increasing energy efficiency of centrifugal chillers.
Such a multi-stage centrifugal refrigerant compressor is generally provided with an economizer cycle. Therein, an economizer is used to reduce liquid refrigerant to liquid saturated pressure and flash off a portion of the refrigerant of inter-stage pressure. The flashed off refrigerant is then mixed with the main-flow refrigerant for next stage compressing. The above process can reduce power consumption of the centrifugal refrigerant compressor and enhance cooling capability per unit of refrigerant. Therefore, multi-stage centrifugal refrigerant compressors have become an only choice for large-scale central air conditioning systems. Along with application of different kinds of refrigerant, high to low pressure ratios during operation of the compressor can be slightly different. Accordingly, there have appeared centrifugal refrigerant compressors having two Stages, three stages or even more stages.
FIG. 1 is a sectional view of a conventional two-stage centrifugal refrigerant compressor. As shown in FIG. 1, the centrifugal refrigerant compressor 1 comprises a high-speed cantilever shaft 19 supported by a bearing. A first stage centrifugal compressor impeller 12 and a second stage centrifugal compressor impeller 16 are connected in series to the overhanging cantilever of the high-speed cantilever shaft 19.
In operation, refrigerant enters into the inlet 11 of the compressor 1 in the direction of R1, and passes through the first stage centrifugal compressor impeller 12, the first stage diffusing flow passage 13, the deswirl vane 14, the return channel bend 15, the second stage centrifugal compressor impeller 16, the second stage diffusing flow passage 17, and the volute casing 18 such that the pressure of the refrigerant can be increased. Thereafter, the refrigerant with high pressure is discharged from outlet (not shown) of the compressor. The centrifugal refrigerant compressor 1 further comprises a refrigerant injection port 110. Gaseous inter-stage pressure refrigerant from an economizer can be injected into a flow passage through the refrigerant injection port 110 in the direction of R2. The refrigerant then passes through the flow passage and comes out from the outlet 111 disposed near the deswirl vane 14. Thereby, the refrigerant from economizer is mixed with the refrigerant in the return channel bend 15 and comes into the second stage centrifugal compressor impeller for further compression.
However, if the refrigerant injected in the direction of R2 and the refrigerant from the deswirl vane 14 can't be mixed uniformly, the pressure, temperature and density will be distributed unevenly at each section of the fluid, thereby adversely affecting the angle of the refrigerant coming into the next stage centrifugal compressor impeller and causing incidence angle difference. Thus, aerodynamic efficiency is reduced.
Increasing the length of the main flow passage between the first and second stage centrifugal compressor impellers, and accordingly increasing the length of the cantilever L1 of the high-speed cantilever shaft will be helpful to overcome the above drawback. However, this method not only reduces the rigidity of the high-speed cantilever shaft, but also increases the flow loss. Meanwhile, size of the compressor in the axial direction is increased. Thus, such problems as efficiency loss and high material cost are generated.
All the above problems are directly or indirectly caused by the one-side discharge of the refrigerant through the outlet 111.
Therefore, there is a need to develop a flow passage structure that can prevent the one-side discharge problem so as to increase efficiency of the centrifugal refrigerant compressor.